Time-locked encryption and time-released encryption seem to be security systems focused on making sure that the decryption of certain message cannot take place before a certain time has passed.

However, I was wondering if there's in cryptography something aiming at the opposite of the above, so I would like to make the decryption possible ONLY during a certain period of time but NOT after that time has passed.

Would it be possible? If so, which mechanism do you know achieving this?

3 Answers
3

Long answer: no, unless there is a trusted person available during the decryption period. And if this is the case the solution is trivial.

Even longer answer: yes, but not through cryptography - you'd need to incorporate physical restrictions. An example would be to send a shuttle outside to space towards some heavy object which will slingshot it back into the general direction of Earth, but headed directly into a star. So the object would have to be (and could only be retrieved) in between the heavy object and the star, or otherwise it would be burned. Another example would be some magical quantum effects I don't know anything about.

This is possible, but only if you introduce one or more parties in addition to the sender and receiver. Also, the receiver must be online when decrypting.

Typically, the security requirements would be that (a) if the recipient tries to decrypt inside the time window, he should be able to decrypt, even if the sender is not cooperating; (b) if the recipient did not decrypt the message inside the time window, he should not be able to learn anything about the contents of the ciphertext; and (c) any additional parties should not learn anything about the ciphertext.

From such requirements, it should be fairly straight-forward to come up with reasonable schemes. Some variant of threshold decryption should do the trick.

As others stated, it wouldn't be possible in an offline sense. Any quirk related to time can be simulated. However if external information is possible, you could have a service secured by some traditional means such as digital signing that provides a one-time pad which is destroyed after the time window is over.